Julias Robert Mayer, a German physicist, born in Heilbronn, Wiirtemberg, Nov. 25, 1814. He received his early education in the gymnasium of Heilbronn, and studied medicine at Tirbingen, finishing his course in Munich and Paris. In 1840 he made a voyage to Java, and spent the summer of that year in Batavia. While there he observed that the venous blood of some of his patients had a singularly bright red color, and he came to the conclusion that it was due to the fact that a less amount of oxidation sufficed to keep up the temperature of the body in a hot climate than in a cold one. The darkness of the venous blood he regarded as the visible sign of the energy of the oxidation. His attention was drawn by this observation to the whole question of animal heat. One great principle of the physiological theory of combustion, he observes, is that under all circumstances the same amount of fuel yields by its perfect combustion the same amount of heat; that this law holds good for vital processes; and that hence the living body is incompetent to generate heat out of nothing. We are thus driven to the conclusion that it is the total heat generated within and without that is to be regarded as the true calorific effect of the matter oxidized in the body.

From this again he inferred that the heat generated externally must stand in a fixed relation to the work expended in its production. He then sought to express this relation numerically. In the beginning of 1842 he had made considerable progress, but having in the mean time become town physician of Heilbronn, he could devote but little time to purely scientific inquiry. He determined however to publish a preliminary notice of the work then accomplished, and he contributed to the May number of Liebig's Anna-len der Chemie und Pharmacie a brief but remarkable paper entitled Bemerkungen iiber die Krdfte der unbelebten Natur, which contained the germ of his future labors. In 1845 he published a memoir entitled Die organische Bewegung in ihrem Zusammenhange mit dem Stoffwechsel, in which he expanded and illustrated the physical principles laid down in his first paper, applying them to organic nature. In 1848 appeared his essay, Beitrdge zur Dy-namik des Himmels, in which he applied the same principles to the heavenly bodies. In 1851 he published another essay, Bemerkungen iiber das mechanische Aequivalent der Wdrme, in which he developed yet further the mechanical theory of heat.

His general argument is that all the mechanical motions upon the earth and all the phenomena of vegetable and animal life are produced by the sun's heat, the source of all power. Nature stores up the light which streams earthward from the sun and casts it into a permanent form. To this end she has overspread the earth with organisms which, while living, take in the solar light, and by its consumption generate forces of another kind. These organisms are plants, and the vegetable world therefore constitutes the instrument whereby the wave motion of the sun is changed into the rigid form of chemical tension, and thus prepared for future use. The physical forces collected by plants become the property of animals. Animals consume vegetables and cause them to reunite with the atmospheric oxygen. Animal heat is thus produced, and also animal motion. Mayer thus grasped the mechanical theory of heat, illustrating it and applying it in the most diverse domains. He began with physical principles; he determined the numerical relation between heat and work; he revealed the source of the energies of the vegetable world, and showed the relationship of the heat of our fires to solar heat. He followed the energies which were potential in the vegetable up to their local exhaustion in the animal.

He then drew attention to the great amount of heat generated by gravity where the force has sufficient distance to act through. He found that the gravitating force between the earth and sun was competent to generate an amount of heat equal to that obtainable from the combustion of 6,000 times the weight of the earth of solid coal. He saw that this was a power sufficient to produce the enormous temperature of the sun, and also to account for the primal molten condition of the earth; and he concludes that the light and heat of the sun are maintained by the constant impact of meteoric matter. Similar conclusions in relation to the mechanical theory of heat were arrived at in England by Dr.'James Prescott Joule almost contemporaneously with the investigations of Dr. Mayer; but there is no reason for supposing that either derived his inspiration from the other. Each was an independent creator of the theory. (See Cokkela-TKm) In the revolution of 1848 Dr. Mayer took what was called the side of order, which aroused against him the antagonism of many of his neighbors. His scientific labors were attacked, and this in connection with the loss of children threw him into an excited and sleepless condition.

On May 28, 1850, being suddenly seized with a tit of delirium, he quitted his bed and leaped from a second-story window, 30 ft. high, to the street below. He recovered from the shock, but his mind was seriously affected. After spending some time in an asylum he was fully restored to health, and he now (1875) resides in Heilbronn. A complete edition of his works has been published under the title Die Meckanik der Wdrme (Stuttgart, 1867). In 1871 the Copley medal was awarded to him by the royal society of London.